In an optical system, when a laser light is emitted from a semiconductor laser oscillator, which is the light source, some of the light becomes reflected light returning from optical elements provided in an optical communication path or the surfaces of devices provided in a partially overlapping optical transmission line or the like, and when the reflected light returns to the laser oscillator, the laser oscillator's performance may become unstable, or the laser oscillator itself may be damaged. So, in order to shield the reflected light off the laser oscillator, an optical isolator is used, and this optical isolator employs a Faraday rotator so as to make use of the latter's Faraday effect.
In general, in an optical communication field of 1200-1650 nm, a bismuth-substituted rare-earth iron garnet which is grown by a liquid phase epitaxial growth method is used as a material to make the Faraday rotator; however this material undergoes material absorption attributable to an ion ingredient, so that in recent years TGG, which undergoes less material absorption, is used. In the case of TGG, a TGG single crystal rod grown by CZ method is used after machining; however, it is difficult to grow a large-sized TGG crystal so that a growth of an ingot of only 1-2 inches in diameter is general, with a resultant problem that the crystal is vulnerable to the influence of deformation induced by machining as well as deformation during the growth.
In addition to TGG single crystal grown based on CZ method, use of a TGG ceramics to make Faraday rotator is being studied as a result of improvement in ceramics technology. Non-IP Publication 1 introduces a TGG ceramics which has similar thermal conductivity and Verdet's constant as a TGG single crystal does, and Non-IP Publication 2 introduces a development of a high quality TGG ceramics having an excellent extinction ratio of ˜40 dB and having similar Faraday effect as a TGG ceramics does. Furthermore, IP Publication 3 describes that a transparent rare-earth gallium garnet sintered compact denoted by a general formula of R3Ga5O12 (wherein R is at least one kind of rare-earth element selected from Y, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu), as the Faraday rotator of an optical isolator.
However, such TGG ceramics sintered compacts are liable to be affected by transmitted beam on account of their grain boundary, and it is pointed out that, similarly as in the case of TGG single crystal, their extinction capability in their material peripheries is degraded owing to the deformation during the material growth, manufacturing or machining stages, with a resulting problem that the isolation (light shielding capability) of the optical isolator is degraded.